This clue is not impossible (7)

I believe the answer is:

soluble

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'impossible' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for soluble that I've seen before include "Capable of being solved or dissolved" , "Like this T 2, one hopes" , "This puzzle should be" , "Dissolvable" , "like most acetates" .)

I've seen this clue in the Evening Standard.

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